An inbound flight, train, or bus is the one coming in to the place you are talking about — arriving, not leaving. At an airport, inbound flights are the ones landing; the ones taking off are outbound. Call centres and warehouses use it too: inbound calls are the ones customers make in; inbound goods are the ones arriving. Though actually, in everyday speech many people just say incoming for calls and arriving for planes; inbound sounds a bit more workplace or travel-industry. Bound here does not mean tied up; it means headed in a direction, the same bound as in homeward bound.